Method of increasing the corrosion resistance of cobalt-based alloys to mercury



United States Patent US. Cl. 117-2 4 Claims ABSTRACT OF THE DISCLOSURE A method of inhibiting the corrosion of an alloy containing cobalt, tungsten, chromium and nickel by hot mercury liquid comprising contacting the alloy with mercury for a time and at a temperature suflicient to leach chromium and nickel out of the alloy to a depth of several mils and annealing the alloy in hydrogen to convert the surface layer to the intermetallic compound COZW.

The invention described herein was made in the course of, or under, a contract with the United States Atomic Energy Commission.

This invention relates to a method of increasing the corrosion resistance of cobalt-based alloys to mercury. In more detail, the invention relates to a method of modifying the surface of wrought, heat-resisting cobalt-based alloys to minimize corrosion by mercury and promote mercury boiling. The invention also relates to a method of reconditioning mercury boilers.

Cobalt alloys such as the alloy given the designation HS-ZS, containing 51% cobalt, 20% chromium, 15% tungsten and nickel plus minor amounts of iron and other constituents, are used in applications Where high temperatures are anticipated. For example, HS-25 appears to be the most suitable material of construction for use in the Mercury Rankine Power Conversion System wherein mercury is used as the working fluid to convert nuclear energy to electrical energy. Obviously, however, anything that will improve its performance would be useful.

It is accordingly an object of the present invention to develop a method of increasing the corrosion resistance of alloys to hot mercury liquid.

It is another object of the present invention to develop a method of increasing the corrosion resistance of alloys containing cobalt and tungsten to hot mercury liquid.

It is still another object of the present invention to develop a method of modifying the surface of cobaltbased alloys to promote mercury boiling.

These and other objects of the present invention are attained by contacting the alloy with mercury at an elevated temperature for an extended time and then annealing the alloy in hydrogen. By the first step certain of the constituent of the alloy are leached out of the alloy to a depth of several mils and by the second step the remaining constituents are converted to an intermetallic compound. For example, in the first Step the mercury leaches nickel and chromium out of HS-ZS and by the second step the cobalt and tungsten remaining are converted to the intermetallic compound Co W by hydrogen annealing.

The time for which the alloy should be in contact with the hot liquid mercury is dependent on the temperature.

The following table gives practical exposures for obtaining a corroded layer of one to three mil thickness.

Temperature, F.: Time, hours 900 2000 1000 1000 1100 500 1200 250 Hydrogen bright annealing is then carried out under a positive pressure with respect to the ambient atmosphere and also at a dew point of minus 60 F. or better and at a temperature of about 2225-2275 F. for about 15 minutes. Lower temperatures, down to 2150 F., would also be satisfactory but would require longer times. The remaining cobalt and tungsten in the leached surfacewhich are present as (Co,W) Creact under the conditions obtaining to form the intermetallic compound COZW which forms a surface coating on the alloy.

A mercury boiler which had operated for 5000 hours at a temperature of 1000 to 1200 F. was successfully reconditioned by hydrogen bright annealing at a temperature of 2250 F. for A-hour under a hydrogen pressure of 15.0 p.s.i.a., as were other mercury boilers which had operated at about the same temperature for shorter periods of time.

To investigate the reason for this successful reconditioning, samples of tubing from another boiler which had been operated for about the same time were cut from the boiler at the point of maximum corrosion. A metallographic examination of an unannealed sample revealed a corrosion layer approximately three mils thick. The ascorroded layer was identified by X-ray diffraction as consisting principally of the M C carbide with a minimal amount (5% or less) of the phase Co W. That the M G carbide is principally (Co,W) C has been confirmed by electron microprobe analysis.

Another sample was hydrogen bright annealed as heretofore described. Photomicrographs of an annealed sample and an unannealed sample indicated that annealing caused a marked change in the appearance of the sample; the fine texture and uniformity of the leached layer of the unannealed sample was replaced by a much coarser structure in which portions of the corrosion layer appear to have been removed; and there was a general agglomeration and sintering of the material which was left.

X-ray diffraction of the corroded and then hydrogen-annealed sample revealed that the layer was now essentially all Co W. The absence of the carbide phase in the hydrogen-annealed corrosion layer indicated the mechanism by which the mercury corrosion-resistant layer was obtained. The M c carbide initially present in the corrosion layer is reduced by the hydrogen:

leaving a composition which assumes the crystal structure of the Laves phase. To confirm this reasoning, corroded sections of HS-ZS tubing were annealed in vacuum and in argon at 2250 F. Since such treatments should not influence the M C carbide, this phase should be present after annealing. This proved to be the case. The X-ray dilfraction results show that M C carbide is present (along with Co W) in both the vacuum-annealed and argonannealed tubing. These tests, then, are confirmation that the M C carbide is reduced by the hydrogen atmosphere annealed at 2250 F. to form the mercury corrosionresistant intermetallic phase Co W.

It is also presumed that hydrogen, a strong reducing agent, removed all of the nitrides and oxides present on and in the surface of the sample during the annealing process and that the vacant spaces caused by this reduction and the agglomeration of the remaining material in the corrosion layer serve as nucleation sites for boiling.

To summarize, this process has the advantage of modifying to a uniform and controllable depth the inside surface of small-diameter tubes shaped into complex configurations; this process has the advantage of forming a surface coating which increases the corrosion resistance of the base alloy; this surface coating is permanent because of its high adherency to the substrate; and the modified surface promotes boiling because of the large number of nuclear boiling sites on the surface.

What is claimed is:

1. A method of reconditioning mercury boilers formed of a wrought, heat-resisting, cobalt-based alloy containing a substantial proportion of chromium, tungsten and nickel which has been in use for a period of time and at a temperature sufficient to leach out the chromium and nickel to a depth of several mils comprising annealing the mercury boilers in hydrogen at a temperature of 2250 F. whereby the remaining cobalt and tungsten in the surface layer are converted to the intermetallic compound Co W.

2. The method of inhibiting the corrosion of wrought, heat-resisting cobalt-based alloys containing a substantial 4 proportion of chromium, tungsten and nickel by hot mercury liquid comprising contacting the alloy with mercury for a time and at a temperature suflicient to leach out the chromium and nickel to a depth of several mils and annealing the alloy in hydrogen for a time and at a temperature sufiicient to convert the remaining cobalt and tungsten in the surface layer to the intermetallic compound Co W.

3. The method of claim 2 wherein the alloy contains 51% cobalt, 20% chromium, 15% tungsten, and 10% nickel.

4. The method according to claim 3 wherein the alloy is contacted with mercury at a temperature of from 900 to 1200" F. for 2000 hours to 250 hours and the annealing is carried out at a temperature of 2225 to 2275 F.

References Cited UNITED STATES PATENTS DAVID KLEIN, Primary Examiner.

US Cl. X.R. 

